In the framework of World Tuberculosis (TB) Day, Dr Bernardo Alonso, renowned WOAH’s expert, shares his vision on the current challenges in the prevention and control of this disease.
Mammalian tuberculosis is a chronic disease caused by bacteria of the Mycobacterium tuberculosis complex that affects cattle, other domestic animals and some wildlife populations. The disease causes general discomfort, pneumonia, weight and production losses.
In humans, most cases of tuberculosis are caused by Mycobacterium tuberculosis which is transmitted from person to person through the air when an infected person coughs, sneezes or speaks, releasing small droplets containing the bacteria.
Zoonotic tuberculosis is a form of tuberculosis in humans caused mainly by the consumption of raw milk or by-products made from raw milk from infected cows or by contact with infected tissues in slaughterhouses or butchers.
In 1882, Robert Koch announced the discovery of the tubercle bacillus that causes human tuberculosis. Later, in 1898, Theobald Smith published his findings on the human and bovine tubercle bacillus, showing that human and bovine tuberculosis were caused by two different organisms known as Mycobacterium tuberculosis and Mycobacterium bovis. ©WOAH/S. Puetata Medina
TB has a zoonotic component, as it can be transmitted to humans, which is enough reason to control it.
In addition, the disease causes economic losses in livestock production, as it decreases productivity by causing lower milk and meat production, as well as higher discard rates of carcasses, half-carcasses and affected organs. This, in turn, reduces the profitability of the sector.
TB also becomes a barrier to international trade, not only in meat and meat products, but also in live animals. Although it is a globally distributed disease, few countries can be considered TB-free, and no country wishes to introduce infected animals into its herds.
In Argentina, it is estimated that 2% of human TB cases are associated with a zoonotic species of the mycobacterium complex, usually Mycobacterium bovis. However, this value may be underestimated, as these cases are not specifically reported.
These cases are often associated with occupational diseases in workers in meat processing plants, slaughterhouses, butchers, veterinarians, or in communities consuming unpasteurised milk or milk products, especially children.
To prevent these cases, it is important to identify and separate animals from TB-positive fields. These animals should receive special treatment during slaughter, implementing specific protection measures.
It is also important to use personal protective equipment in slaughtering plants, such as gloves, masks and appropriate footwear.
To prevent cases arising from the consumption of dairy products, pasteurisation is key. Pasteurisation has been a key factor in reducing the prevalence of zoonotic tuberculosis. In Argentina, it is compulsory for all dairy products to be pasteurised. In other countries with low prevalence, some by-products derived from unpasteurised milk are allowed under strict controls.
Pasteurisation is compulsory in most countries of the Americas.
The guidelines for the control of Mycobacterium tuberculosis complex in livestock provide recommendations on key aspects for the control of these infections in livestock addressing the challenges faced by countries with diverse epidemiological, economic and socio-cultural characteristics. ©WOAH/A. Cipriani Schwengber
Diagnostic tests are done on the live animal. However, the inspection of visible lesions compatible with tuberculosis at slaughter plants, together with traceability to the herd of origin, is an essential tool in any disease control and eradication plan.
The most widely used test is the tuberculin or PPD (purified protein derivative) test, which has been performed for more than 70 years. This test measures delayed immunity and detects whether the animal has been in contact with the bacteria at some point in its life.
Studies and pilot tests on the use of vaccines are also underway. However, one of the disadvantages of this methodology is that it can interfere with the diagnosis, as it does not allow differentiation between vaccinated and infected animals. Nevertheless, vaccination might help to reduce the prevalence of the disease over time, especially in areas where a test-and-slaughter strategy cannot be implemented.
The main challenge is the lack of accurate data on the prevalence of the disease, as the test-and-slaughter strategy is costly and not applied in all herds. In addition, bovine TB is not always perceived as a serious problem by farmers, as infected animals usually do not show visible symptoms and can be replaced without major problems. It becomes a moral responsibility of the farm owner towards his employees because of the zoonotic nature of the disease.
There is also a high under-reporting of cases, as TB is a chronic disease that does not always leave visible lesions during slaughter, and routine inspection of animals does not always detect signs of the disease.
Another challenge is that we do not have techniques that have a high certainty of diagnosis, there are always positive animals inside that are not identified by the tuberculin test. However, this is a global challenge that is not limited to the Americas region.
The World Organisation for Animal Health (WOAH) publishes international standards that serve as guidelines on the techniques to be used for the diagnosis and control of tuberculosis, as well as to ensure the safety of international trade in animals and animal products. These standards are regularly updated to ensure that they are scientifically rigorous.
WOAH also evaluates new diagnostic techniques through the registration of new kits. A laboratory has the possibility to register its product with WOAH. The organisation then shares it with experts to evaluate that product under different conditions and issue their report.
Currently, a WOAH’s Ad hoc group is working on a replacement of the reference PPD reagent due to depletion of the current one.
WOAH, the World Health Organization (WHO), the Food and Agriculture Organization of the UN (FAO) and the International Union Against Tuberculosis and Lung Disease (The Union) jointly launched the first-ever roadmap to tackle zoonotic TB in 2017.
WOAH reference laboratories provide support in the evaluation of new reagents and diagnostic kits, provide training in the regions to which they belong, produce reference sera and provide general advice on the management and control of this disease, among other activities.
For countries with limited resources, I would recommend a long-term plan. Eradicating bovine TB is not a quick process, but it is possible with proper planning. It is crucial to know the prevalences, individualise animals, use appropriate reagents and have trained staff.
It is also important to maintain a good relationship with public health. The ‘One Health’ collaboration is key to raising public awareness and preventing zoonotic cases through livestock or livestock products.
